The invention concerns a device for processing a signal delivered by a radiation detector. The invention also concerns a radiation detecting system that comprises a radiation detector and device for processing the signal delivered by the detector according to the invention.
The invention more particularly concerns the detection at ambient temperature of ionizing electromagnetic radiation (x rays, gamma rays) using semiconductors such as, for example, CdZnTe, CdTe:Cl, CdTe:In.
The use of semiconductors as ionizing electromagnetic radiation detectors has a number of advantages:                a direct conversion of the electromagnetic radiation into an electric signal [gamma/light/electric charges] when a scintillator is used associated with a photomultiplier),        detectors operating at ambient temperature, therefore not very bulky, and a high electric signal making it possible to obtain excellent energy resolutions.        
For several years, progress relative to the growth of the materials and technology for the detectors as well as progress in electronics and processing of the signal have made it possible to confirm the ambitions of semiconductors in ionizing electromagnetic radiation spectrometry and greatly open their fields of use, namely, for example: 2D imagers for medical imaging with the development of CdZnTe-based scanners, baggage checking systems using x-rays in the security field, nuclear probes for testing irradiated assemblies.
The field of use of the present invention is, more particularly, but not exclusively, that of examining luggage using x-rays, in particular to detect the presence of explosives.
A synoptic schema of an ionizing electromagnetic radiation detection system is symbolically illustrated in FIG. 1. The detection system comprises an ionizing electromagnetic radiation detector 1, an electronic proximity circuit 2 that measures the signal delivered by the detector 1, an electronic processing circuit 3 (filtering, amplifier . . . ) that processes the signal delivered by the electronic circuit 2 and an electronic system 4 that digitizes the signals delivered by the electronic circuit 3 in order to obtain the histogram of those signals.
Electromagnetic radiation spectrometry thus consists of counting and measuring, with the greatest possible precision (energy resolution), the energy of all of the photons making up the electromagnetic radiation and that are absorbed by the detector (detection efficiency).
Depending on the fields of use, the incident electromagnetic radiation is made up of a more or less numerous photon energy spectrum with a very variable energy range (several keV to several MeV). The choice of thickness and surface of a semiconductor detector depends on the detection efficiency and expected sensitivity of the system, respectively. As a non-limiting example, a thickness smaller than a millimeter is sufficient to stop the great majority of the low-energy photons (<100 keV), but, for high-energy photons (>100 keV), a thickness of several millimeters, or even a centimeter, may be necessary.
A significant thickness detector makes it possible to stop a maximum of photons (increasing the interaction probability), but decreases the possible counting rate because the transit time of the charges in the detector is directly proportional to its thickness. A high bias voltage, as well as particular configurations of the detector (irradiation by section), then make it possible to increase the efficiency while maintaining a very low transit time (current pulses<100 ns).
As previously mentioned, an ionizing electromagnetic radiation spectrometry system comprises, aside from the detector 1, an electronic proximity circuit 2, an electronic processing circuit 3 and a digitization circuit 4.
FIG. 2 shows an electronic proximity circuit 2 connected to a detector 1. FIG. 3 shows the current i(t) that is delivered by the detector and enters the electronic circuit 2 and FIG. 4 shows the voltage VOUT (t) delivered by the electronic circuit 2.
The detector 1 comprises a block of semiconductor material M and a resistor R that connects the block M to a high voltage HT. The electronic proximity circuit 2 is a charge preamplifier that comprises a capacitor C1, an amplifier A1, a capacitor C2 and a resistor Rp. The capacitor C1 is mounted at the inlet of the amplifier A1 and the capacitor C2 and the resistor Rp are mounted in series between the inlet and outlet of the amplifier A1.
Upon detection of the interaction of a photon ph, a current i(t) can be collected by an electrode connected to the detector. During the presence time of the detecting current i(t) delivered by the detector 1, the voltage VOUT (t) output from the charge preamplifier is:
                    V        OUT            =                        -                      1                          C              ⁢                                                          ⁢              2                                      ⁢                  ∫                      i            ⁢                          ⅆ              t                                            ,                  ⁢    or              V      OUT        =          -              Q                  C          ⁢                                          ⁢          2                    where Q is the quantity of charges emitted by the photon that interacts in the semiconductor material M (cf. temporal zone Za in FIG. 4).
In output from the charge preamplifier, the information corresponding to the energy of the photon is fleeting because the preamplifier unloads. It is therefore necessary to save this voltage as quickly as possible after the detecting current disappears (cf. temporal zone Zb in FIG. 4). In parallel, the relaxation of the charge preamplifier makes it possible to face high counting rates, since the output voltage therefrom accumulates and, without relaxation, the preamplifier would quickly be saturated (cf. the saturation voltage Vsat (in FIG. 4)).
The voltage VOUT (t) delivered by the electronic circuit 2 is the input voltage of an electronic processing circuit 3. FIG. 5 shows the voltage Vs (t) delivered by an electronic processing circuit 3 as a function of time.
The electronic processing circuit 3 comprises a band-pass filter that makes it possible to optimize the signal to noise ratio. A number of impulse filters can be used, filters with n derivations and n integrations, Gaussian filters, trapezoidal, triangular, digital, etc. These filters are often matchable and it is possible to adjust the derived and integral times to best adjust the signal to noise ratio in the band of interest. For all of these filters, the aim to be achieved is to have a pulse in their outputs whereof the amplitude is proportional to the energy of the photon that interacts in the detector. It has been noted that impulse filters alter the temporal information corresponding to the duration of the detector current; it is increased by the by-pass and the integration. This effect decreases the admissible counting rate by increasing the pile-ups. It is recalled that the counting rate corresponds to the number of pulses detected at the terminals of the detector per unit of time.
When the counting rate is high, the voltage Vs(t) output from the filter does not have enough time to return to zero, the amplitude of the voltage of the following photon is then wrong.
Another significant drawback of the filter comes from its poor linearity when it is used for detectors that deliver current pulses having large shape variations (thick detectors), this is the ballistic deficit error.
The usual solution to reduce this error consists of filtering with a time constant much higher than the duration of the pulse of the input current, which is obviously in conflict with rapid shaping of the pulses adapted to high counting rates.
FIG. 6 shows a switched integrator able to measure the energy of the detected photons, i.e. the amplitude of the pulses output from the band-pass filter. The integrator comprises a first switch SW1, a resistor Ri, an amplifier AMP, a capacitor Ci and a second switch SW2. The first switch SW1 and the resistor Ri are mounted in series, the resistor Ri being placed at the input of the amplifier AMP. The capacitor Ci and the second switch SW2 are mounted in parallel between the input and output of the amplifier AMP. FIGS. 7 and 8 show, respectively, the signal Vs(t) (output voltage of the filter) that enters the integrator and the signal y(t) that comes out of the integrator. The entering signal Vs (t) has an amplitude Am.
Before the signal Vs(t) from the impulse filter arrives, the switch SW1 is open and the switch SW2 is closed. Once the signal Vs(t) crosses a threshold voltage the switch SW2 opens and the switch SW1 closes. Between moments t0 and t1, the signal Vs(t) is integrated. As of moment T1, threshold passage moment on the pulse edge of the signal Vs(t), the switch SW1 opens. As output from the peak detector, the signal Y(t) has an amplitude proportional to the input pulse (|Y(t)|=k Am), therefore to the energy of the photon.
Between moments t1 to t2, the output signal is kept constant, allowing it to be saved in a data system. As of moment t2, the switch SW2 closes: the detector is reset and the system can process a new photon.
This type of switched integrator is used in rapid spectrometry chains with high counting rates, i.e. in electronic spectrometry circuits adapted to detectors subject to intense radiation and producing numerous pulses per unit of time, or high counting rates. A counting rate is generally considered high beyond some 100,000 photons detected per second (or counts per second). The integrators previously described can be used up to several Megaphotons/s. For even higher counting rates, the use of this type of integrator becomes difficult, in particular due to the switches SW1 and SW2, the response times (switching delay between the command and the analog output) of SW1 and SW2 then no longer allowing a complete integration of the signal Vs(t), thereby causing an error on the measurement of the photon's energy. Moreover, the resistance of the switch SW2 in the transition state occurs during the reset time of the integrator, which also contributes to slowing down the measurement chain.
To be able to correctly conduct measurements by spectrometry when a detector is subjected to high counting rates, another solution exists in the prior art. It involves measuring the energy of the photons using a delay line. FIG. 9 shows a counting system using a delay line. Such a system does not use a filtering circuit and the output of the charge preamplifier 2 here is directly connected to an assembly formed by a delay line Lr, an attenuator Att (gain less than 1), a subtractor D, an amplifier A2 and an analog/digital conversion circuit ADC. The delay line Lr is mounted in series with the attenuator Att and forms a delay and attenuation block whereof a first terminal is connected to the output of the preamplifier and the second terminal is connected to a first input of the subtractor D whereof the second end is directly connected to the output of the preamplifier.
The signal VOUT(t) from the preamplifier is delayed through the delay line Lr, the delay of which is greater than the rise time of the signal VOUT(t). The subtractor D subtracts the delayed and attenuated voltage VOUT(t) from the voltage VOUT(t) and the signal resulting from that subtraction is amplified by the amplifier A2, which then delivers, via the amplifier A2, a pulse E(t) whereof the height is proportional to the pulse produced at the terminals of the detector, i.e. the energy ceded by the photon detected in the detecting material. The digitization done by the analog/digital converter ADC is done continuously, the computer being programmed to pick up energy values above a predetermined energy threshold ES. Once the energy values are picked up, more or less elaborate algorithms calculate the corresponding energy value of the photons. FIG. 10 shows an example of a detected energy curve E(t) as a function of time. The points distributed on the curves E(t) symbolically illustrate the digitization of the signal E (t) that is done by the analog/digital converter ADC.
The emission of the photons obtained with an x-ray generator or a radioactive source is a random emission. It is then necessary to process the coincidences, i.e. the periods of time where numerous photons are emitted in a very short time interval, thereby contributing to a piling up of signals as input of the analog/digital conversion circuit. It is specified that the more intense the radiation to which the detector is subject, the higher the probability of such coincidences.
A number of methods for processing coincidences are known. One of the simplest consists of measuring the width of the pulse (time between two successive threshold crossings) and comparing that value with a reference period. If the pulse is too long, there is a pile-up and the measured energy value is not taken into account and the pulse is rejected. The following photon is then awaited. The processing method has the drawback of not being very efficient, in particular when the counting rate is high, in which case there are many pile-ups; most of the pulses then have a duration exceeding the reference duration and are rejected. Thus, the system's efficiency becomes mediocre, such an efficiency representing a ratio between the number of pulses processed and the number of pulses detected.
Also known from the prior art is a device for processing and digitizing an energy spectrum of an electromagnetic radiation such as the device disclosed in patent application EP 2 071 722 published on Jun. 17, 2009. This device comprises a preamplification circuit, a pulse measuring circuit with delay line, a sampler, a current pulse measuring circuit making the difference between the output signal of the preamplification circuit and a derivative of the output signal of the preamplification circuit, and a discrimination circuit producing a binary signal as a function of the output signal of the current pulse measuring circuit, said binary signal controlling the sampling moments of the sampler. This device makes it possible to correct the detection problems related to the random nature of the sampling and the generation of charges in the ionizing electromagnetic radiation spectrometry detection systems.